The present invention relates to aircraft electronics and in particular to aircraft weather radar systems.
Weather radar uses echo returns from transmitted radio signals (typically in the megahertz and gigahertz spectrum) to locate and characterize precipitation and its motion. Precipitation may be characterized by the intensity of the echo return and motion characterized by Doppler principles in which moving particles provide a frequency shift to the echo return.
Aircraft weather radar uses these principles to provide guidance to pilots with respect to storm cells and the like. In such systems, a radar antenna may be fixed to the nose or wing of the aircraft and scanned either mechanically or using phased array techniques. Typically the radar beam is collimated to a focused ray which is scanned in a horizontal plane directed outward along the flight path of the aircraft to provide information about weather in the path of the aircraft, while reducing reflections from the ground (ground clutter). The angle of the radar beam with respect to the horizon may be adjusted, however, so that the pilot may direct the radar to regions of interest not necessarily at the current altitude of the aircraft.
The weather data derived from the echo returns may be displayed in real-time on a display in the cockpit. The image on the display is normally displayed in 2 dimensions and oriented with respect to a small fixed aircraft icon so that weather systems directly in front of the aircraft are positioned directly above the icon while those to the left and right of the aircraft are depicted to the left and right of the icon respectively. This approach provides the pilot a constant indication of weather patterns in front of the aircraft flight path.
While mechanisms for moving radar antenna or adjusting the angle of a phased array can theoretically scan the field of view of the radar at high speed, as a practical matter the need to wait for echo returns and sufficient data sampling, limit the scan rate of the antenna to on the order of seven seconds per scan acquisition.
Unlike a ground-based radar, the antenna used in aircraft weather radar is mounted to a moving object (e.g. the aircraft) which can be subject to constant changes in orientation particularly if there is buffeting in storm regions. It is known that some weather radar systems stabilize the angle of the scanning plane of the radar using a vertical gyroscope on the aircraft.